Electron discharge tube with crossed electric and magnetic fields



D. CHARLES ELECTRON DISCHARGE TUBE WITH CROSSED ELECTRIC AND MAGNETICFIELDS Dec. 18, 19.56

2 Sheets-Sheet 1 Filed May 28, 1952 Dec. 18, 1956` D. CHARLES 2,774,913

ELECTRON DISCHARGE TUBE WITH CROSSED ELECTRIC AND MAGNETIC FIELDS FiledMay 28, 1952 2 Sheets-Sheet 2 United States ELECTRON DISCHARGE TUBE WITHCROSSED ELECTRIC AND MAGNETIC FIELDS Daniel Charles, Paris, France,assignor to Compagnie Generale de Telegraphie Sans Fil, Paris, France, aFrench company The present invention relates to electronic dischargetubes having crossed electric and magnetic fields, and more particularlyto a new and improved electron gun for use in such tubes.

The general object of the invention is to provide a tube of this typewherein a stream of electrons is caused to follow a substantiallyrectilinear path between two electrodes across which an electricpotential is maintained. One of the prerequisites for such operatingconditions is generally that the cathode emitting the electrons be quitesmall in the direction of travel of the electrons, a plurality ofcathodes of similar size being used as an alternative to enlarging thecathode, when it is desired to increase the emission.

The amount of current which may be applied to the cathode or cathodesis, however, 'limited by the sensitivity of the cathode to currents ofhigh density, the life of the cathode being inversely proportional tothe strength of the current, and the latter being further limited byother considerations even for a short life of the cathode.

It is, therefore, a further object of this invention to provide anelectron gun in a tube of the type described wherein the cathode maybeof substantially greater size than heretofore without thereby impairingthe operation of the tube.

With these and other objects in "view,the invention may be defined `ascomprising an electron gun for electron discharge tubes having crossedelectric and magnetic fields including a cathode of large dimension inthe direction of movement ofthe electrons.

More specifically, the emissive cathode is situated in the plane of asubstantially iiat electrode, and lit emits electrons into a spacebounded on the yone side by the at electrode and on the other sidev byan electrode having a hyperbolic prolile decreasing kkinthe desireddirection of movement of the electrons, the latter electrode beingraised to a positive potential relatively to the flat electrode. Thespace between the electrodes is traversed vby a magnetic fieldperpendicular to the plane `in which the electrons areemitted, with aView to imparting curvature to the path of the electrons in the desireddirection. Furthermore, this space is divided into two zones by a gridraised to an electric potential intermediate the potentials-of the twoelectrodes, the extent of the grid being limited to a predetermineddistance 'lengthwise of the cathode in the direction of movement of theelectrons, At the forward edge of the grid, thehyperbolic electrode iscontinued by a substantiallyiiat electrode co-extensive of a furtherfiat electrode raised to the potential of the first at electrode `anddening therewith a further space which is not divided into two Zones bythe grid. The potentials to which the electrodes and the grid areraised, the magnitude of the magnetic field, the transverse distancesbetween various electrodes and the longitudinal distance between theforward edge of the grid and the cathode are suitably selected so thatthe electric field in the undivided space is equal to 'the field betweenthe grid and the hyperbolic electrode at the level of the' ice forwardedge of the grid, and so that the electrons pass through the grid at avery4 small angle relatively thereto, for example about 1-3", and,thereafter, pass into the undivided space without again contacting thegrid.

In the accompanying drawings:

Figures 1 4 are diagrammatic illustrations of the operating conditionsand the dimensions'of an electron gun according to the present inventionand Figure 5 is a longitudinal section through a travelling wave tube,including an electron gun constructed and arranged in accordance withFigures l-4. l

The invention will be more readily understood with reference to Figurel, which illustrates diagrammatically a section of the gun in a planeperpendicular to the direction of the magnetic field. In Figure l theiiat electrode l is raised, for example to a potential O and it containsan electron emissive cathode 4. A grid 2, which is permeable .to theelectrons is located at a transverse distance d from the electrode 1 andit is raised to alpotential V. In the direction of movement of theelectrons, the extent of `the grid is limited to a distance 1- rneasuredfrom the rear edge of the cathode 4. An electrode 3, raised to apotential V2 comprises a rst portion of hyperbolic profile merging intoa second portion of rectilinear profile positioned at a distance di froma further flat electrode 5 which is at the same potential as theelectrode 1, the electrode 5 being at a distance y1 from theequipotential V1 along which the path 6 of the electrons issuing fromthe rearward edge vof the cathode 4 becomes substantially rectilinearand the same electrode 5 being at a distance yo from the grid 2. Amagnetic field is applied in a directionA perpendicular to the plane ofFigure -1 as indicated at H, and the effect of this field is to bend thepath of the electrons toward the right if the potentials Vo and V2 ofthe grid 2 and of the electrode 3 are positive.

The operation of the electron gun constructed according to Figure l willbe readily understood from a step by step -description of Figures 2-4,

Figure 2 shows an electrode l at a potential O positioned at a distanced from an electrode 2 permeable to the electrons i. e. a grid, raised toa potential Vo, and at a distance di-d from the electrode 2 there is anelectrode 3 raised -to a potential V2, the three electrodessconstituting a plane parallel system.

The cycloidal movement of an electron issuing from the plane of theelectrode 1 in a uniform electric eld `E and in a magnetic field h` issuch that the field, at the highest point of the trajectory is:

El v== whence E=2E (2) If in the system according to Figure 2, therelationship is Vini VVVO d 2 dl-z (s) the condition is fulfilled, andthe electro-ns issuing from kthe points A, B, or C will follow cycloidalpaths in the zone I, and rectilinear paths parallel to the equipotential`Vo in the rzone v`II.

lf the arrangement according to Figure 2 could :be-materialized inphysical terms, it would enable the use of 3 a cathode of infinitelength, the width being the dimension of the cathode parallel to thedirection of movement of the electron beam.

In order that the electrons may pass effectively through the grid,however, and enter the zone II, they must approach the grid at aninclination of a few degrees. In such circumstances the movement of theelectrons in the zone II will no longer be along a rectilinear path. Thepath will be trochoidal on the contrary, and the electrons will fallback onto the grid.

Figure 3 illustrates the same elements as Figure l. In the Zone I, theelectron will follow a first path a of cycloidal shape, and in the zoneII it will follow a second path b of elongated trochoidal shape, passingagain through the grid at c, and following a further cycloidal path gidentical to a.

If the separation into two zones I and Il is suppressed slightly beforethe point c in the plane XY, and if the further zone III to the right ofthe plane XY is governed by an electric field equal to the field in thezone II, the trochoidal path will extend along the line e, the amplitudeof the oscillations being proportional to the angle of incidence of theelectrons passing through the grid 2 at the point D.

In order that the electrons as they pass through the grid may form withthe plane of the grid an angle other than zero, and in order to preventthe electron from falling back onto the grid, the electric field in theZone II must increase slightly in the direction of movement of theelectron. This field must be so selected, however, that the amplitude oftransverse movement remains relatively low.

It has been found that the field defined by the electrode 3 in Figure l,which comprises hyperbolic equipotentials, is characterized byparticular properties perfectly suited for carrying out the presentinvention.

In the system shown in Figure 4, an electrode 2 corresponding to a gridof the previous figures is maintained at a potential Vu, and anelectrode 3 of hyperbolic shape having an equation Jcy=h2 is maintainedat a potential V2. The electrode 2 extends along an asymptote Ox of thehyperbola 3.

If an electron enters into the system of Figure 4 at a velocity vexpressed in electron volts by the value Vo and at an angle a, it willbe found that there is a dimension x0 measured from the intersection Oof the asymptotes of the hyperbola 3, according to which the electronentering the zone II at that point follows a hyperbolic path.

The distance xo is defined by the following formula:

At a distance x1 measured from the same origin O, the electron will besituated on an equipotential V1, such that The hyperbolic electrode at apotential V2 is defined by its constant h2 as follows:

Since a is small it is known from Formula 6 that the amplitude remainssmall even for a cathode of large surface.

0,177.H2. tan a In order to adapt the space between the electrodes 2 and3 to the discharge space, it is assumed that in the discharge spaceconstituted by'two parallel planes between which there is maintained aconstant electrical field, the electron must travel along anequipotential in such a manner that the velocity of the electronexpressed in volts be exactly the potential value of this equipotential.

If the adaptation is made at a distance x1 from the origin, the velocityof the electron in electron volts is V1. One of the properties of thefield defined as above is that at each point, if a is small, thequotient of the transverse component of the electrical field by themagnetic field H is exactly equal to the velocity of the electronexpressed in electron volts as V1, and for this reason the adaptation tothe discharge space is particularly easy.

It will be understood from the foregoing explanations that the structureof the electron gun shown in Figure l is particularly well designed. Forcalculating the elements of the gun the origin data y1, V1 and di areconventional and the first step is to calculate:

V1d1 V 8 2 y1 f hence,

wherein e and m are the charge and the mass of the electron.

In practice, angle a is very small, in the neighborhood of l-3 degrees.It is then possible to determine the values of Vo, d, h, x1, x0 and Yofrom the Equations 4, 5 and 7 to which three further equations are addedas follows:

The final Equation l2 is not compulsory but it corresponds to aparticularly favorable dimensioning of the parts for carrying out theinvention.

The width of the cathode s is determined from the Equation 6 inaccordance with the maximum permissible thickness of the beam Finallythe distances x1 and x2 being obtained as set forth above, and thehorizontal distance between A and D, equal to the projection of a halfsegment of a cycloid of amplitude d being determined by it is possibleto obtain the equation which gives the exact location of the rearwardedge A of the cathode 4 relatively to the plane of adaptation XY asfollows:

f=7r2l+ixo (13) The theoretical construction and practical calculationof the electron gun according to the present invention being fully setforth, it is possible to examine an example of use of the gun in atraveling wave amplifier having crossed magnetic and electric fields.Such a tube is illustrated in Figure 5 wherein the same electrodes areshown as in Figure 1 with the same reference numerals.

An electrode 1 containing a cathode 4 at the gun end of the tube iselectrically connected to an electrode 5 at the interaction space end`of the tube where the beam interacts with the wave. Lead b connects theelectrodes 1 and 5 through the envelope 8 having a socket 13. The grid 2is also provided with a lead a passing through the socket 13 in orderthat the grid may be raised to a positive potential relatively to theplane of the cathode, lead a being connected to a battery 17. Thehyperbolic electrode 3 has a lead issuing through socket 14 which raisesthe electrode to a positive potential through the same battery 17.

The cathode 4 of relatively large surface is heated by an element havingconnections c and d passing through the socket 13 and connected to abattery 18. The paths of two typical electrons are shown at 6 and 7.

A delay line 9, parallel to the electrode 5, is provided with input andoutput connections 10 and il for supplying the electromagnetic wave andrespectively extracting the amplified wave. The tube is completed by acollecting electrode 16 having a lead issuing through socket 12 forconnection to the same potential source as the electrode 3. The .tube isplaced in a magnetic tield perpendicular to the plane of Figure andoriented in the direction in which the gure is viewed.

As previously pointed out, a tube of this construction has the advantageof enabling the use of a cathode of relatively large dimension with veryaccurate focusing, but it is also possible to cut off or re-establishthe ow of electrons through the grid 2 by a very slight variation in theamplitude of Vo. It will be seen indeed that the Equation is identicalwith that which gives the critical iield, the cosine being very closeto 1. A very slight decrease of V0 will prevent the electrons frompassing through the a second electrode comprising a first portion ofhyperbolic profile having its convex surface facing the emissive surfaceof said cathode and a second portion of average rectilinear profileextending from the rst portion, a grid separating the space between thefirst and second electrodes, said grid extending from a zone ofthe tubeadjacent the cathode to a zone of the tube adjacent the junction betweenthe rst and second portions of the second electrode, means for applyinga positive potential to the second electrode relatively to the firstelectrode, 4means for applying to the grid a potential intermediatethose applied to the first and second electrodes, and means forproducing a magnetic field having lines of force perpendicular to theaxis `of the tube and to the lines of force of the electric fieldresulting from the difference of potential between the rst and secondelectrodes.

2. A tube according to claim 1 wherein the second portion of the secondelectrode comprises a delay line having input and output extremities,means being provided for supplying a wave to the input extremity and forcollecting an amplified wave from the output extremity.

References Cited in the file of this patent UNITED STATES PATENTS2,413,251 Smith Dec. 24, 1946 2,414,121 Pierce c- Jan. 14, 19472,566,087 Lerbs Aug. 28, 1951 2,607,904 Lerbs Aug. 19, 1952 2,654,004Bailey Sept. 29, 1953 2,680,823 Dohler et al. June 8, 1954 OTHERREFERENCES Article by Warnecke, Doehler and Bobot, pp. 279-291, Annalesde Radioelectricite, October 1950.

